Quantum bicriticality in the heavy-fermion metamagnet YbAgGe

Bicritical points, at which two distinct symmetry-broken phases become simultaneously unstable, are typical for spin-flop metamagnetism. Interestingly, the heavy-fermion compound YbAgGe also possesses such a bicritical point (BCP) with a low temperature T_BCP ~ 0.3 K at a magnetic field of mu_0 H_BCP ~ 4.5 T. In its vicinity, YbAgGe exhibits anomalous behavior that we attribute to the influence of a quantum bicritical point (QBCP), that is close in parameter space yet can be reached by tuning T_BCP further to zero. Using high-resolution measurements of the magnetocaloric effect, we demonstrate that the magnetic Grueneisen parameter Gamma_H indeed both changes sign and diverges as required for quantum criticality. Moreover, Gamma_H displays a characteristic scaling behavior but only on the low-field side, H < H_BCP, indicating a pronounced asymmetry with respect to the critical field. We speculate that the small value of T_BCP is related to the geometric frustration of the Kondo-lattice of YbAgGe.Comment: submitted to PR

Competing Magnetic Fluctuations in Iron Pnictide Superconductors: Role of Ferromagnetic Spin Correlations Revealed by NMR

In the iron pnictide superconductors, theoretical calculations have consistently shown enhancements of the static magnetic susceptibility at both the stripe-type antiferromagnetic (AFM) and in-plane ferromagnetic (FM) wavevectors. However, the possible existence of FM fluctuations has not yet been examined from a microscopic point of view. Here, using $^{75}$As NMR data, we provide clear evidence for the existence of FM spin correlations in both the hole- and electron-doped BaFe$_2$As$_2$ families of iron-pnictide superconductors. These FM fluctuations appear to compete with superconductivity and are thus a crucial ingredient to understanding the variability of $T_{\rm c}$ and the shape of the superconducting dome in these and other iron-pnictide families.Comment: 5 pages, 4 figures, accepted for publication in Phys. Rev. Let

Direct observation of Fe spin reorientation in single crystalline YbFe6Ge6

We have grown single crystals of YbFe6Ge6 and LuFe6Ge6 and characterized their anisotropic behaviour through low field magnetic susceptibility, field-dependent magnetization, resistivity and heat capacity measurements. The Yb+3 valency is confirmed by LIII XANES measurements. YbFe6Ge6 crystals exhibit a field-dependent, sudden reorientation of the Fe spins at about 63 K, a unique effect in the RFe6Ge6 family (R = rare earths) where the Fe ions order anti-ferromagnetically with Neel temperatures above 450 K and the R ions' magnetism appears to behave independently. The possible origins of this unusual behaviour of the ordered Fe moments in this compound are discussed.Comment: 12 pages, 8 figures, accepted in J. Phys.: Cond. Matte

Magnetic ordering in GdNi2B2C revisited by resonant x-ray scattering: evidence for the double-q model

Recent theoretical efforts aimed at understanding the nature of antiferromagnetic ordering in GdNi2B2C predicted double-q ordering. Here we employ resonant elastic x-ray scattering to test this theory against the formerly proposed, single-q ordering scenario. Our study reveals a satellite reflection associated with a mixed-order component propagation wave vector, viz., (q_a,2q_b,0) with q_b = q_a approx= 0.55 reciprocal lattice units, the presence of which is incompatible with single-q ordering but is expected from the double-q model. A (3q_a,0,0) wave vector (i.e., third-order) satellite is also observed, again in line with the double-q model. The temperature dependencies of these along with that of a first-order satellite are compared with calculations based on the double-q model and reasonable qualitative agreement is found. By examining the azimuthal dependence of first-order satellite scattering, we show the magnetic order to be, as predicted, elliptically polarized at base temperature and find the temperature dependence of the "out of a-b plane" moment component to be in fairly good agreement with calculation. Our results provide qualitative support for the double-q model and thus in turn corroborate the explanation for the "magnetoelastic paradox" offered by this model.Comment: 8 pages, 5 figures. Submitted to Phys. Rev.

Effect of proton irradiation on the normal state low-energy excitations of Ba(Fe1−x_{1-x}Rhx_x)2_2As2_2 superconductors

We present a \asnmr Nuclear Magnetic Resonance (NMR) and resistivity study of the effect of 5.5 MeV proton irradiation on the optimal electron doped ($x=$ 0.068) and overdoped ($x=$ 0.107) Ba(Fe$_{1-x}$Rh$_x$)$_2$As$_2$ iron based superconductors. While the proton induced defects only mildly suppress the critical temperature and increase residual resistivity in both compositions, sizable broadening of the NMR spectra was observed in all the irradiated samples at low temperature. The effect is significantly stronger in the optimally doped sample where the Curie Weiss temperature dependence of the line width suggests the onset of ferromagnetic correlations coexisting with superconductivity at the nanoscale. 1/T$_2$ measurements revealed that the energy barrier characterizing the low energy spin fluctuations of these compounds is enhanced upon proton irradiation, suggesting that the defects are likely slowing down the fluctuations between ($0,\pi)$ and ($\pi$,0) nematic ground states.Comment: 9 pages, 9 figure

Single crystal growth and characterization of the large-unit-cell compound Cu13Ba

Single crystals of Cu$_{13}$Ba were successfully grown out of Ba-Cu self flux. Temperature dependent magnetization, $M(T)$, electrical resistivity, $\rho(T)$, and specific heat, $C_p(T)$, data are reported. Isothermal magnetization measurements, $M(H)$, show clear de Haas-van Alphen oscillations at $T$ = 2 K for applied fields as low as $\mu_0H$ = 1T. An anomalous behavior of the magnetic susceptibility is observed up to $T$ ~ 50K reflecting the effect of de Haas-van Alphen oscillations at fairly high temperatures. The field- and temperature-dependencies of the magnetization indicate the presence of diluted magnetic impurities with a concentration of the order of 0.01at.%. Accordingly, the minimum and lower temperature rise observed in the electrical resistivity at and below $T$ = 15K is attributed to the Kondo impurity effect.Comment: 6 pages, 8 figures, accepted for publication in J. Alloys Comp

Drastic annealing effects in transport properties of single crystals of the YbNi2B2C heavy fermion system

We report temperature dependent resistivity, specific heat, magnetic susceptibility and thermoelectric power measurements made on the heavy fermion system YbNi2B2C, for both as grown and annealed single crystals. Our results demonstrate a significant variation in the temperature dependent electrical resistivity and thermoelectric power between as grown crystals and crystals that have undergone optimal (150 hour, 950 C) annealing, whereas the thermodynamic properties: (c_p(T) and chi(T)) remain almost unchanged. We interpret these results in terms of redistributions of local Kondo temperatures associated with ligandal disorder for a small (~ 1%) fraction of the Yb sites.Comment: 5 pages, 4 figures, submitted to PR

Angular dependent planar metamagnetism in the hexagonal compounds TbPtIn and TmAgGe

Detailed magnetization measurements, M(T,H,theta), were performed on single crystals of TbPtIn and TmAgGe (both members of the hexagonal Fe_2P/ZrNiAl structure type), for the magnetic field H applied perpendicular to the crystallographic c axis. These data allowed us to identify, for each compound, the easy-axes for the magnetization, which coincided with high symmetry directions ([120] for TbPtIn and [110] for TmAgGe). For fixed orientations of the field along each of the two six-fold symmetry axes, a number of magnetically ordered phases is being revealed by M(H,T) measurements below T_N. Moreover, T ~ 2 K, M(H)|_theta measurements for both compounds (with H applied parallel to the basal plane), as well as T = 20 K data for TbPtIn, reveal five metamagnetic transitions with simple angular dependencies: H_{ci,j} ~ 1/cos(theta +/- phi), where phi = 0^0 or 60^0. The high field magnetization state varies with theta like 2/3*mu_{sat}(R^{3+})*cos(theta), and corresponds to a crystal field limited saturated paramagnetic, CL-SPM, state. Analysis of these data allowed us to model the angular dependence of the locally saturated magnetizations M_{sat} and critical fields H_c with a three coplanar Ising-like model, in which the magnetic moments are assumed to be parallel to three adjacent easy axes. Furthermore, net distributions of moments were inferred based on the measured data and the proposed model